Fractures of the long bones, such as the femur, are often treated with the use of an intramedullary rod (“IM rod”) inserted into the medullary canal of the affected bone. An IM rod, as is well known in the art, generally comprises an elongated rod along with associated cross-members, such as screws or nails, including nailing devices with helical blades. The IM rod typically includes various transverse holes to allow for the placement of these associated cross-members through the IM rod and into bone tissue in order to stabilize and hold together the fragmented bone segments. For instance, in the treatment of fractures in the area of the neck and/or head of the femur, a cross-member can be inserted through the proximal portion of the IM rod, across the fracture, and then into the femoral head. For more distal shaft type fractures, locking screws can be placed through the IM rod and into bone tissue at appropriate locations to provide fixation of the bone fragments.
A technique for implanting intramedullary rods involves insertion of the rod through a point that is lateral to the center-line of the medullary canal, i.e. offset from the piriformis fossa. One of the many examples of the use of this technique is illustrated in U.S. Pat. No. 3,433,220 to Zickel. An osteotomy at the tip of the trochanter is made to create an entry site and a flexible reamer is utilized to carry out the reaming of the medullary canal while conforming to its basic anatomy. Because the location of the entry site is laterally offset from the axis of the medullary canal, an angled or curved opening is created between the entry site and the medullary canal.
Once the opening is made, an IM rod can be inserted through the entry site and into the medullary canal. However, the insertion point of an IM rod in a laterally offset placement is the site of possible iatrogenic fractures because the curvature of the opening into the medullary canal “angles” the rod into the medullary canal of the femur. This causes the potential for fracture due to the inadvertent application of transverse point loads against the bone. Fractures have been observed beginning at the entry site and extending through the intertrochanteric region following IM rod insertion.
In addition, the shape of the medullary canal of the femur itself can complicate the insertion of an IM rod. The medullary canal has a gentle, uniform, anterior bowing throughout its length. If the IM rod does not possess an anterior bend or curvature, transverse point loads may act against the cortical wall of the femur leading to fracture. Over-reaming the medullary canal can prevent fracturing of the shaft during insertion, but this results in a decrease of the surface area contact between the rod and the bone, leading to less effective fixation of the bone fragments. Another detrimental effect of over-reaming is a decrease in the bending and torsional strength of the bone. As a result, many IM rods have anterior bends or curves in the anterior-posterior plane to match the normal medullary anatomy of the femur, as exemplified in the prior art by patents such as U.S. Pat. No. 3,433,220 to Zickel and U.S. Pat. No. 4,135,507 to Harris.
However, curvature of the rod in the anterior-posterior plane alone does not necessarily overcome the difficulties arising from the insertion of the IM rod through the laterally offset entry site. Additional side point loads may be imposed on the bone by the proximal segment of the IM rod once the rod is in its final position because the anterior curvature of the rod is in an orthogonal plane to the curvature of the opening between the entry site and the medullary canal. This may also result in subsequent fracturing of the femur. As a result, some IM rods (such as disclosed in the prior art Zickel and Harris patents) have incorporated a bend or curve in the lateral-medial plane that attempts to conform with the opening from the entry point into the medullary canal. Nevertheless, these laterally curved rods have not been completely successful in eliminating inadvertent fracturing during the insertion or removal procedure.
When an IM rod is provided with an anterior curve, rotating the rod approximately 90 degrees about its longitudinal axis prior to insertion may facilitate its insertion into the medullary canal because the rod's curvature could more closely approximate the curvature of the opening between the entry site and the medullary canal. Thus, the rod is initially inserted in this rotated orientation and then twisted as it is driven into the medullary canal to its final position. However, applying the proper amount of twisting force at the appropriate time is problematic as it may be difficult to continuously monitor the precise extent of the rod's progress into the medullary canal while also applying the corresponding amount of twist required at each point. In addition, IM rods are often provided with external flutes extending straight down the surface of the shaft of the rod that may interfere with this twisting motion during insertion. While these external flutes are desirable because they provide benefits such as improved medullary revascularization, reduced stiffness with greater strength, and improved torsional fixation at the rod-bone interface, the engagement of the flutes with bone inside the medullary canal may actually impede the twisting necessary in order to insert the rod.